There are many man-made structures, such as pipelines, offshore platforms, buildings, bridges, dams and the like for which structural monitoring is important to verify design codes, test loadings and forewarn of potential or actual failures of the structure or parts thereof. For example, it is known to place strain gauges, microphones for acoustic emissions, tilt meters using accelerometers and the like along a structure, such as a marine riser or pipeline to provide signals indicative of strains or deflections beyond normal limits.
Movements in natural objects, such as earth strata or rock formations adjacent to an earthquake fault, are more difficult to measure because of inhomogeneity. Seismic, tilt meter or land surveying techniques are consequently employed to measure acceleration, tilting or displacement, respectively.
While the use of strain gauges or other measurement means periodically spaced along a structure will serve to aid in providing the desired strain data, only those discrete points on the structure to which the strain gauge is secured are monitored. There may be other locations on the structure sufficiently spaced from the point of attachment of the strain gauge or other sensor which will have on influence thereon and yet experience a physical movement or stress which could be significant to the safety or potential failure of the structure.
In addition, power must be supplied to such prior art sensors and data must be acquired, all of which involves cable, power and telemetry equipment, plus associated logistic and maintenance support. The cost for such a system can become excessive and reliability can become impaired because of the number of elements involved.
Attempts have been made to overcome this difficulty by the development of structural frequency-measurement systems placed at a central point on a structure. The technique employs Fourier analysis to detect modal shifts in frequency resulting from changes in structural integrity, such as fractures in members or even loss of members. In the case of an offshore platform, however, the variations in loading on the structure, non-linearity of the foundation (piling) and inconsistency of natural excitation have precluded sufficient signal to noise ratio to render such a system feasible for identifying the location of structural changes.
In view of the foregoing, there is a need for an improved method and apparatus for monitoring some structures wherein all points along the structure, or between designated parts of a structure, can be "continuously" and reliably monitored. By such a "continuous" arrangement, location of structural movement could be determined, there being no gaps in the monitoring system.